Frequency discriminating circuit



f 2/0 cATHoDe cATHoDE Els FoLLoweR FoLLowER Feb. 12, 1952 V. R. BRIGGS FREQUENCY DISCRIMINATING CIRCUIT Filed Jan. 26, 1948 l2 4 -'l E; /2/ I @22 FOLLOWER CATHODE INVENTOR VERNON R. BRIGGS ATTORNEZ Patented Feb. 12, 1952 UNITED STATES 'PATENT OFFICE FREQUENCY 'DISCRIMINATING CIRCUIT Xernonlt. Briggs, Pasadena, Calif., assigner-.to Bendix Aviation-Corporation, .South Bend,v Ind., ancor-porationof Delaware Application January 26, A1948,' Serial'No.4,425 Claims. (Cl. Z50-27.)

This "invention relate-s.: "to frequency n'xodulaition'systems, and more particularlyto frequencyrresponsive circuits, dr `frequency Vdiscrirninators `asthey are more commonly referred to, for produclng a. direct currentvarying in amplitude in `accordance kwith frequency variations of :an A. C. :signal wave.

rcriminator circuit that has a 'predictable lfrequency range and requires few adjustments 'itherat thezfactory or inthe iield.

Another object is 'to provide a frequency disfcriminator-that'canbe fabricated from standard parts.

'.Othermorespecicobjectsand features o the invention will becomeapparent from the descrip- `tion'to follow of certain embodiments thereof.

The present frequency diseriminator` differs essentially from other frequency discriminators knownto mein-thatin .placeof .the usual resonant circuitsitemploys what is known asia. twin .T network consisting .solelyof resistors andca- .pacitors, one T-.oi the network comprising two resistors in .series with a shuntcapacitor `between `them, and the other T comprisingtwo capacitors .in series with each other and in parallel with the Atwo first mentioned resistors, the shunt kelement ofthe other. T ccnsistingpo a resistor. The values of thecapacitorsand resistors are prescribed only to the extentthat the shunt capacitor must have twicethe value of each ofthe series capacitors, v'and' the'shunt resistor must have half the resistancev of'each `of theI series resistors. Such a net- 4Worlrhaszero outputat aeriticalirequency fo, whichifrequency is 'equalto .1v v'zrR `where .R..represents the value of each seriesresistorand C..represents the value of each series capacitor. The .twin .T .network has the further -.useful .characteristic that when the frequency risesabove the critical frequencythe output increases substantially linearly andthe phase of output wave is advanced nearly 90 relative to v the input wave, whereas when the frequency is reduced below the critical frequency, the ampli- .tude'of the output also increases linearly with the departure in' the' frequency, but the `phase 'of' the output wave lags the input wave by'nearly 90.

2 vIn :accordancewi-th the present invention the input of Aa twin T' network asdescribed has applied thereto the frequency modulated signal wave that is tobe. detected, andits output isccmzbined vwith other potentials derived from the signal .voltage, I and the-resultant rectified, the-rarrangementbeing such that the rectifledoutput current has zero Value when the frequency of the signal Wave is equalto .the critical frequency or the twin T network, but the rectified current increases -.in amplitude with any departure .from .the critical frequency .in either direction, being ,oi

.onepolarity when the frequency .departureis Yin vOnedirection, and of the opposite polarity when the frequency departure is inthe opposite Vdirection.

The new circuit has substantial A advantages .over `conventional yfrequency ,discriminating circuits,.because the heart oftheV system is thetwin T network consisting.r only of. resistors and capacitors which arereadil-y-obtainable in accurately gauged sizes such .thatthey Vcan becornputed in `advance Vto give a known performance with no A.adjustment necessaryafter assembly. The sys- 1-temis particularly useful for Adetecting frequency modulatedwaves,ofrelatively low frequency, be-

cause.the.-resistorandcapacitor values are within readily obtainable ranges.

Another advantageous feature 4or my circuit is that itis possible tocompensatefor thelosses of the vtwinj' network .byY means. of a simple ccnventional A. C. amplifier, `and thereby eliminate the necessity for-employing a D. C. amplifier. This vis desirablebecause of the relative 4instal.bility of D. C...arnpliiiers. YAnother factor con- .trilouting .to the `stability of ,the discriminatorie the. fact that nov coupledresonant circuits are employed. Itis recognized that the adjustment yoi coupled circuits. is often vvery critical.

The present inventionisuseful in a variety of .systems `where `frequency variations of an alter- .nating current .are to be detected.

Inthe drawing: Fig. 1 is a highly schematic circuit showing a lsimple form of the` invention;

`ligs. 2,3,.and .4 .are vector diagrams illustrating: the operation ofthe circuit of Figi;

.Fig.- .5 A`is a .graph illustrating the electrical --characteristicsoi atwin T network; and

Fig. Sisaschematic circuit showing. an actual 4circuit incorporating .the invention that may be employed.

Referring first to Fig. 1.a frequency-modulat- 'ed A. C. waveo potential Es is 'applied to a pair lof -input terminals' l which are connected vin parallel to the input of a phase shifter I I and a twin T network I2. The output of the phase shifter II is shifted 90 with respect to the input and is connected to the primary winding ill of a transformer I 5 having a, mid-tapped secondary winding consisting of two sections I6 and I1 respectively, connected in series.V The outer ends of the secondary windings I6 and I1 are interconnected through a rectifier I8, load resistors I9 and 22, and a rectifier 2I. The load resistors IS and 22 are shunted by filtering condenser-s 23 and 24 respectively.

The output of the twin T network I2 is connected in series in the common return path from the junction ISI of the load resistors I9 and 22 back to the mid-tap of the secondary windings I5 and i7. This twin T network I2 consists of a pair of series resistors I2I and I22 and a shunt capacitor |23 constituting one T network, and a pair of series capacitors |24 and I25 and a shunt resistor |26 constituting the other T network.

As has been previously indicated, at a critical or null frequency, the output of a twin T network is zero and the value of the null frequency fo may be determined from the formula:

In practice fn is chosen as the center frequency of the variable frequency signal wave Es to be detected. When the signal wave is at the null frequency fo, the potentials present at various points in the circuit of Fig. 1 are as shown in the vector diagram of Fig. 2. In this diagram, the phase of the signal voltage Es is chosen as the reference value in zero position. The potential in the output circuit of the phase shifter II is shifted 90, so that the potential E2 across the transformer winding I B is shown in Fig. 2 displaced in 90 leading relation with respect to Es. The potential E4 developed in the secondary section I 7 is displaced 180 from the potential E2 as indicated in Fig. 2. Under the conditions described, the output of the twin T network I2 is zero. Therefore E5 is zero and the potentials developed across the two load resistors I9 and 22 are equal, and the potential Ex between the output terminals and 2E is zero.

The vector diagram of Fig. 3 represents the condition when the frequency of the signal wave is greater than fu. Under this condition, the potentials Ez and E4 are the same as before, but the twin T network I2 now develops an output wave E5 and this wave is advanced in phase substantially 90 with respect to the input wave Es and is therefore substantially in phase with the potential En. This potential E5 is therefore added to E2 and subtracted from E4, producing a higher potential at terminal 25 than at terminal 2S, the potential difference being a measure of the departure of the frequency of the signal wave from a null value in one direction.

Fig. 4 represents the potentials in the circuit when the frequency of the signal wave is less than the null frequency fo. Under these conditions, the twin T network I2 develops an output potential E5 that lags the input or signal potential ES. This potential E5 therefore is in aiding relation with E4 and opposing relation with E2, producing a potential difference between the terminals 25 and 25 that is again proportional to the departure of the frequency of the signal wave from the null frequency fo, but is of opposite polarity to that resulting from the conditions of Fig. 3.

In actual practice, the basic circuit of Fig. 1 may be modified as shown in Fig. 6, which differs essentially from the circuit of Fig. 1 in the use of: (l) cathode followers introduced at various points in the circuit for the purpose of matching impedances and improving stability; (2) the use of two 45 phase Shifters in the two branches of the circuit instead of a single 90 phase shifter in only one branch; and (3) the use of an A. C. amplier in the circuit branch containing the twin T network to compensate for the losses inherent in the network.

Thusit will be observed that in Fig. 6 the input terminals l0 are connected directly to the input circuit of a cathode follower 30, the output circuit of which contains two R. C. circuits 3| and 32 connected in parallel. The circuit 3| contains a resistor 3H and a capacitor 3 I 2 with the capacitor connected to the grounded side of the output of the cathode follower 30. The other R. C. circuit 32 comprises a capacitor 32| and a resistor 322 with the lat-ter connected to the grounded side of cathode follower 30. A second cathode follower 33 has its input circuit connected across the resistor 322 and its output circuit connected to the primary winding I4 of the transformer I5. Athird cathode follower has its input circuit connected across the capacitor 3I2 and its output circuit connected to the input circuit of the twin T network I2. The output circuit of the twin T network i2 is connected through an amplifier 35 and a fourth cathode follower 30 to the midtap of the secondary winding of the transformer I5.

The potential across the capacitor 3l 2 lags the output potential of the cathode follower 30 by approximately whereas the potential across the resistor 322 leads the output potential by about 45, thereby effecting a total phase shift of approximately between the potential waves delivered to the cathode followers 33 and 34 respectively.

The amplifier 35 is designed to compensate for the losses introduced by the twin T network I2 and to deliver a potential through the cathode follower St to the mid-tap 20 that is substantially equal to the voltage Ez or E4 under conditions of maximum departure of the frequency of the sig- .nal current from the null frequency.

To overall characteristics of the circuit shown in Fig. 6 are approximately as illustrated in Fig. 5, in which the curve Q0 represents the magnitude and polarity of the D. C. potential Ex between the output terminals 25 and 2E with variations of the signal frequency below and above the null frequency fu. It will be observed that over a frequency range of approximately 10% below and above the null frequency, the output voltage Ex varies substantially linearly, as shown by the straight, sloping portion 00a of the curve 40. The frequency range over which the response is linear can be varied by varying the gain of the amplifier 35. Thus, the curve becomes flat, as indicated at db and 40e when the magnitude of the potential E5 equals that of E2 and E4, and the magnitude of the frequency departure required to make E5 equal E2 and E4 depends upon the amplification introduced by the amplifier 35.

Although for the purpose of explaining the invention, a particular embodiment thereof has been shown and described, obvious modifications will occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

I claim:

1. The method of detecting frequency variations in a variable frequency A. C. signal wave which comprises: deriving from said signal wave second and third A. C. waves of the same frequency but differing from each other in phase substantially 90 throughout the operating frequency range; deriving from said signal wave a fourth A. C. wave 180 out of phase ywith said second wave; deriving from said third wave a fifth A. C. wave of the same frequency that has zero amplitude at a null frequency and that increases in amplitude with departures of frequency in either direction from said null value, and the phase of which with respect to the third wave leads substantially 90 for all amplitudes when the frequency exceeds said null value and lags substantially 90 for all amplitudes when the frequency drops below said null value; combining said second and fifth waves and rectifying them to produce a rst direct current; combining said fourth and fifth waves to produce a second direct current; and combining said first and second direct currents in polar opposition to produce a direct current the amplitude of which varies with the frequency of said signal wave.

2. Frequency discriminating apparatus comprising; means for deriving from an A. C. signal wave second and third A. C. waves of the same frequency but differing from each other in phase by substantially 90 throughout the operating frequency range; means for deriving from said signal wave a fourth A. C. wave 180 out of phase with said second wave; means for deriving from said third wave a fifth A. C. wave of the same frequency that has zero amplitude at a null frequency and that increases in amplitude with departure of frequency in either direction from said null Value and the phase of which leads that of the third wave substantially 90 when the frequency exceeds said null value and lags that of the third wave substantially 90 when the frequency drops below said null Value; means for combining said second and fifth waves and rectifying them to produce a rst direct current;

means for combining said fourth and fifth waves and rectifying them to produce a second direct current; and means for combining said first and second direct currents in polarity opposition to produce a direct current the amplitude of which varies with the frequency of said signal wave.

3. Apparatus according to claim 2 in which said means for deriving said fifth wave comprises a twin T network having zero output at said null frequency, said twin T network having separate input and output terminals, said third wave being applied to said input terminals, and said second and fourth waves being applied through said output terminals to said rectifying means.

4. Apparatus according to claim 2 in which said means for deriving said fifth wave has an output impedance that is substantially without reactance over the operating frequency range whereby said fifth wave is combined with said second and fourth waves without altering the phase of the latter.

5. Apparatus according to claim 2 in which said means for deriving said third and fourth waves comprises a non-resonant impedance network and an untuned transformer, and said means for deriving said fifth wave has an output impedance that is substantially without reactance over the operating frequency range whereby said fifth wave is combined with said second and fourth waves in phase with one or the other of the latter throughout the operating frequency range.

VERNON R. BRIGGS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,323,609 Kihn July 6, 1943 2,415,468 Webb Feb. 11, 1947 2,495,023 Sebring et al. Jan. 1'7, 1950 

